A solid polymer type fuel cell is a system fueled using pure hydrogen, hydrogen gas obtained by conversion of alcohol, etc. and electrochemically controlling the reaction between the hydrogen and the oxygen in the air to take out power.
A solid polymer type fuel cell uses a solid hydrogen ion selective permeation type organic membrane as an electrolyte, so can be made more compact compared with a conventional alkali type fuel cell, phosphoric acid type fuel cell, molten carbonate type fuel cell, solid electrolyte type fuel cell, or other fuel cells using as an electrolyte an aqueous solution-based electrolyte, molten salt-based electrolyte, or other fluid medium. Development is underway for application to electric vehicles etc.
The configuration of a typical solid polymer type fuel cell is shown in FIG. 1. A solid polymer type fuel cell 1 is configured by a solid polymer membrane 2 forming an electrolyte, a catalyst electrode part 3 comprised of carbon fine particles and precious metal ultrafine particles provided on the two surfaces of this solid polymer membrane 2, a current collector comprised of a felt-like carbon fiber composite having the function of taking out power generated at this catalyst electrode part 3 as current and supplying the catalyst electrode part 3 with the reaction gas, that is, the oxygen-containing gas or hydrogen-containing gas (usually called “carbon paper 4”), and a separator 5 receiving current from the carbon paper 4 and separating oxygen-containing gas and hydrogen-containing gas stacked together.
The basic principle of the solid polymer type fuel cell 1 is generally as follows: That is, in a solid polymer type fuel cell 1, fuel comprised of hydrogen gas (H2) 8 is supplied from the anode side 6, passes through a gas diffusion layer comprised of the carbon paper 4 and the catalyst electrode part 3, and forms hydrogen ions (H+) which permeate through an electrolyte comprised of the solid polymer membrane 2. At the catalyst electrode part 3 at the cathode side 7, the hydrogen ions (H+) and the oxygen (O2) in the air 9 supplied from the cathode side 7 undergo an oxidation reaction (2H++2e−+1/2O2→H2O) whereby water (H2O) is produced. At the time of this oxidation reaction, electrons 10 generated at the catalyst electrode part 3 of the anode side 6 flow through the carbon paper 4 from the separator 5 of the anode side 6 to the separator 5 of the cathode side 7, whereby current and voltage are generated between the two electrodes.
The solid polymer membrane 2 has an electrolyte having a strong acidity fixed in the membrane and functions as an electrolyte allowing permeation of hydrogen ions (H+) by control of the dew point in the cell.
The separators 5, component members of the solid polymer type fuel cell 1, have the function as channels of separating the two types of reaction gases, that is, the air 9 at the cathode side 7 and the hydrogen gas 8 at the anode side 6, and supplying the reaction gases and the function of discharging the water produced by the reaction from the cathode side 7. Further, in general, the solid polymer type fuel cell 1 uses a solid polymer membrane comprised of an electrolyte having a strong acidity. Due to the reaction, it operates at a temperature of about 150° C. or less. Water is produced, so the solid polymer type fuel cell separators 5 are required to have corrosion resistance and durability as material properties and are required to have a good conductivity for allowing current to efficiently pass through the carbon paper 4 and a low contact resistance with the carbon paper.
In the past, as the material of the solid polymer type fuel cell separators, carbon-based materials have been used in large amounts. However, separators comprised of carbon-based materials cannot be made thinner due to the problem of brittleness, so obstruct compactness. In recent years, separators made of hard-to-break carbon-based materials have been developed, but they are expensive cost-wise, so are disadvantageous in economic terms.
On the other hand, separators using metal materials do not have the problem of brittleness compared with carbon-based materials, so in particular enable a solid polymer type fuel cell system to be made more compact. Separators using low cost materials such as stainless steel or titanium or a titanium alloy or other metal materials are being developed. Numerous proposals are being made (see, for example, Japanese Patent Publication (A) No. 2000-260439, Japanese Patent Publication (A) No. 2000-256808, Japanese Patent Publication (A) No. 2004-107704, Japanese Patent Publication (A) No. 2004-156132, Japanese Patent Publication (A) No. 2004-273370, Japanese Patent Publication (A) No. 2004-306128, Japanese Patent Publication (A) No. 2004-124197, Japanese Patent Publication (A) No. 2004-269969, Japanese Patent Publication (A) No. 2003-223904, Japanese Patent Publication (A) No. 2004-2960, and Japanese Patent Publication (A) No. 2004-232074).
However, stainless steel separators or titanium or titanium alloy separators have the problem of a larger contact resistance with carbon paper due to the passivation film formed on their surfaces and therefore a large drop in the energy efficiency of the fuel cell.
For this reason, in the past, numerous proposals have been made for methods for reducing the contact resistance between the surfaces of members of stainless steel separators or titanium and titanium alloy separators and carbon paper.
For example, a separator for a solid polymer type fuel cell reduced in contact resistance with carbon paper by forming a large number of bulging parts on the surface of stainless steel (SUS304) by press forming and forming gold plating layers of predetermined thicknesses on the end faces at the front end sides (see, for example, Japanese Patent Publication (A) No. 2004-265695) or depositing a precious metal or precious metal alloy on the stainless steel or titanium surface (see, for example, Japanese Patent Publication (A) No. 2001-6713) has been proposed. However, these methods require gold plating or other surface treatment forming expensive precious metal layers in order to impart conductivity to the stainless steel or titanium surface, so had the problem of an increase in the production costs of the separator.
On the other hand, various methods have been proposed to reduce the contact resistance between the surface of the separator members and carbon paper while reducing the amounts of the expensive precious metals used or eliminating their use.
For example, in order to reduce the contact resistance between the stainless steel surface and carbon paper, the method of causing the Cr in stainless steel to precipitate as chromium carbides in the annealing process of the stainless steel and raising the carrying capacity of the current received from the carbon paper through the chromium carbides exposed on the surface of the passivation film formed on the stainless steel surface (see, for example, Japanese Patent Publication (A) No. 2000-309854) and the method of providing the stainless steel surface with a coated film in which SiC, B4C, TiO2, or other conductive compound particles are dispersed, then heating this stainless steel in a nonoxidizing atmosphere to 300 to 1100° C. to cause the main ingredients of the coated film to break down and dissipate or coating the surface with a carbide-based conductive ceramic to form the conductive compound particles on the stainless steel surface (see, for example, Japanese Patent Publication (A) No. 11-260383 and Japanese Patent Publication (A) No. 11-219713) are known. However, these methods require a step of long heat treatment to form a conductive compound on the stainless steel surface, so had the problems of a drop in productivity of the separator and increase in production costs. Further, with the method of causing the Cr in stainless steel to precipitate as chromium carbides in the annealing process, when in particular the annealing time is not sufficient, a chrome-deficient layer will form near the chromium carbides in the steel, a local drop in corrosion resistance will occur in this region, and, when press forming the stainless steel to form a gas channel on the separator surface, the chromium carbides may form starting points for cracks in the stainless steel surface.
Further, methods of fixing a carbon layer or carbon particles with a good conductivity on the stainless steel surface have also been proposed. For example, the method of forming a gas channel by press forming etc. the important part where the catalyst electrode is located on the metal sheet, then forming a carbon-based conductive coating layer on that surface (see, for example, Japanese Patent Publication (A) No. 2000-021419), the method of improving the conductivity by dispersing and press bonding carbon powder at the stainless steel surface (see, for example, Japanese Patent Publication (A) No. 11-121018), and the method of forming an Ni—Cr-based plating layer or Ta, Ti, or Ti—Ta-based plating layer in which carbon-based particles are dispersed on the stainless steel surface (see, for example, Japanese Patent Publication (A) No. 11-126621 and Japanese Patent Publication (A) No. 11-126622) are known. However, in the separators made by these methods, due to the pseudo Schottkey barrier formed at the carbon side in the electron structure at the interface of the metal and carbon, a large contact resistance is caused at the interface of the stainless steel and carbon layer or carbon particles and as a result the effect of sufficiently reducing the contact resistance with the carbon paper cannot be obtained.
Further, the method of forming one or more types of conductive ceramic layers of TiN, TiC, CrC, TaC, B4C, SiC, WC, ZrN, CrN, and HfC at the fuel electrode side supplying hydrogen-containing gas in a stainless steel separator (see, for example, Japanese Patent Publication (A) No. 2003-123783) has been proposed. This method forms a conductive ceramic layer by vapor deposition using a vacuum apparatus etc. or dry coating etc., but has the problems of limits to the film-forming speed and an unavoidable drop in the yield of the coated substance, so an increase in the production costs.
Further, the method of fixing hard fine powder having conductivity on the substrate surface by shot etc. has been proposed.
For example, a titanium or titanium alloy separator at the substrate surface of which are buried, dispersed, and exposed conductive hard particles of the M23C6 type, M4C type, or MC type, where the metal element (M) is at least one of chrome, iron, nickel, molybdenum, tungsten, and boron (see, for example, Japanese Patent Publication (A) No. 2001-357862), and stainless steel and a stainless steel separator in the substrate surface of which are buried, dispersed, and exposed conductive hard particles of at least one of the M23C6 type, M4C type, M2C type, and MC type carbide-based metal inclusions and M2B type boride-based metal inclusions, where the metal element (M) is at least one of chrome, molybdenum, and tungsten, and having a surface roughness of a center line average roughness Ra of 0.06 to 5 μm (see, for example, Japanese Patent Publication (A) No. 2003-193206) have been proposed.
Further, a method of spraying a separator forming a fuel cell with a solid plating material comprised of core particles of a higher hardness than this separator and coated with a metal having a high corrosion resistance and a low contact resistance with carbon so as to forcibly deposit the metal coated on the solid plating material on the separator (see, for example, Japanese Patent Publication (A) No. 2001-250565) and a method of using a similar technique to bury a fine amount of a precious metal in stainless steel or titanium or a titanium alloy to obtain a sufficiently low contact resistance even without coating the entire surface with a precious metal like with gold plating (see, for example, Japanese Patent Publication (A) No. 2001-6713) have been proposed.
The method of fixing hard fine powder having conductivity on the substrate surface by shot etc. is more advantageous than the methods of heat treatment or vacuum deposition in that it does not lower the productivity, is cheaper in product costs, and is simpler as a method. On the other hand, with the method of mechanically burying hard conductive particles in the substrate surface of a metal separator formed into a desired shape by shot etc., distortion may be introduced into the substrate surface layer part resulting in deformation and the flatness of the separator may drop.
In general, a solid polymer type fuel cell has a low output voltage per cell of about 1V, so to obtain the desired output, a large number of fuel cells are often stacked for use as a fuel cell stack. For this reason, in the method of fixing hard fine powder having conductivity on the substrate surface by shot etc., it is necessary to treat the separator under conditions suppressing the occurrence of warping or distortion in the separator and giving a good flatness enabling the fuel cells to be stacked.
Further, the lower the contact resistance of the separator with the carbon paper, the more desirable. For example, a method of depositing metal on a fuel cell separator characterized in that the value of the low contact resistance with the carbon is not more than 20 mΩ·cm2 at a contact surface pressure of 1 kg·f/cm2 (see, for example, Japanese Patent Publication (A) No. 2001-250565) etc. have been proposed.
In the above way, in the past, metal separators for solid polymer type fuel cells using, as the separator substrate, a metal material superior in corrosion resistance such as stainless steel or titanium or a titanium alloy and improving the contact resistance between the separator substrate surface and carbon paper by using various methods to form a conductive compound layer on the substrate surface or fix conductive compound particles on the surface have been proposed, but these could not be said to have been necessarily sufficient from the viewpoints of the contact resistance and flatness demanded from a solid polymer type fuel cell separator or from the viewpoints of productivity and production costs.